This invention relates to systems for tracking projectiles using optical means.
On release of a ground or air launched guided missile which is intended to intercept an aerial target, for example, it is necessary to track the missiles position relative to the target up to the point of impact.
It is known to use a scanned laser beam to illuminate a friendly missile and thereby track the missile. For example, in GB-A-2,113,939 receivers on board a missile deduce the missile""s position relative to a ground-based tracking system by decoding a modulation superimposed on the scanning beam. This method requires sophisticated electronic circuitry to be carried onboard the missile for the purpose of decoding the modulation. A further disadvantage is the lack of a communications link from the missile back to the tracking system.
This invention aims to provide a two-way communications link between a tracking station and a friendly missile, the link being realizable using comparatively simple components.
According to the present invention a tracking system comprises:
a transmitter and a receiver, the transmitter including means for producing a scanned optical beam for the illumination of a projectile;
and a plurality of transponders for mounting on the projectile, each transponder comprising a detector/emitter pair for detecting the scanned beam and for emitting a further optical beam in response thereto for reception by the receiver.
The means for producing a scanned optical beam may comprise a laser coupled to a mechanical scanning arrangement (eg. as described in our co-pending Application EP92301208 published as European Patent Application publication number 50026B) or to an acousto-optic deflector cell.
An acousto-optic deflector cell may comprise any suitable material which exhibits an acousto-optic effect ie. diffraction of light by acoustic waves. One example of such a material is tellurium dioxide. Acoustic waves are usually coupled into the cell via a piezoelectric transducer, for example, which is bonded to one face of the cell.
As is known, an acousto-optic deflector cell may be configured to receive a beam of laser light, of frequency f say, and in response to a high frequency drive signal applied to the cell (in the MHz to GHz range), some of the light emerging from the cell is deflected to form a so-called xe2x80x9cfirst order beamxe2x80x9d. The angle of deflection of this beam with respect to the undeflected zero order beam is substantially proportional to the frequency of the drive signal, (fac) which initiates a sound wave which propagates across the cell. Hence by varying the drive signal frequency in some controllable manner, a beam may be scanned in a single plane. Two dimensional scanning may be achieved by incorporating a second deflector cell through which the first order beam produced by a first cell is arranged to pass as described in GB-A-2,113,939.
The transponders may comprise a single solid-state photodetector element located adjacent to a laser diode. Preferably the divergence of the beam emitted by the laser diode is small in order to minimize the risk of battlefield eye damage.
Some embodiments of the invention will now be described, by way of example only, with reference to the drawing which is a schematic representation of a tracking system in accordance with the invention.